(a) Field of the Invention
This invention relates to a printer, and in particular to a printer having a head capable of imprinting characters on to paper--or a like medium--suitably supported adjacent a path along which the head is traversed to print a line of characters.
(b) Description of the Prior Art
The head in a printer of the aforesaid type may take one of several forms. For example, the head may be a generally spherical element provided with characters at known positions on the spherical surface, the element being mounted for both rotating and tilting movement to align a required character with the paper, whereafter the head is driven on to the paper to imprint the character thereon. Another type of head has a disc or "daisy wheel" with characters either around the rim of the disc or on the ends of the "petals" (or spokes) of the wheel, the disc or wheel being rotated to bring a required character to lie adjacent the paper. A hammer, forming a part of the head, is located on the opposite side of the paper to the disc or wheel and is traversed therewith; at each character location, when the required character on the disc or wheel faces the paper, the hammer strikes the paper to drive it on to the character, thereby leaving an impression thereon. In yet another type of head, a plurality of needles are arranged in a substantially vertical line (relative to the paper) and are fired electro-magnetically into contact with the paper in accordance with a preselected pattern for any particular character, a pre-set number of vertical lines being used to make up any given character. Typically, seven needles are used, there being a maximum of five vertical lines per character--that is, the character is made up from a 7.times.5 matrix. Another array often used is a 13.times.10 matrix. A modification of this type of head uses electrostatic discharge needles and a heat-sensitive paper, a charge being given to the required needles to burn a mark on the paper at the required position so as to build up a character.
In all the above forms of printer, the head is traversed across the paper as a line of print is made up. This is usually effected by means of a servo-motor fed with appropriate signals and winding a cord fastened to the head against a spring force; the fly-back traverse to the start of a line is then achieved by the action of the spring, when the servo-motor is let free.
When printing speeds are increased, considerable difficulties arise in printing the characters exactly where required. For high speed isochronous operation, the characters can be printed "on the fly"--i.e. without stopping the traverse of the head, but when operating in the stop/start mode, the inertia of the head becomes a serve problem. From rest, when a character is to be printed, the head must be accelerated to a high speed in case a second character rapidly follows the first, but if no second character immediatly follows, the head instantly must be stopped. The inertia prevents very high accelerations and decelerations being obtained, and this in turn leads to unequally-spaced characters--which makes the print commercially unacceptable. Similarly, when a line is finished and the head has to perform the fly-back traverse, the first character of the next line must fall immediately below that of the line above, but the positioning of the head is difficult because of inertia. A ragged left-hand margin often occurs because the head position for the first character of a line is defined by a stop for the head, but at high speeds the head inevitably bounces from the stop to some extent at the end of the fly-back traverse.